Triple Duplicate ACK

First one must understand what a duplicate ACK is.

Duplicate ACK

RFC 5681 gives the following definition:

DUPLICATE ACKNOWLEDGMENT: An acknowledgment is considered a "duplicate" in the following algorithms when

1. the receiver of the ACK has outstanding data,
2. the incoming acknowledgment carries no data,
3. the SYN and FIN bits are both off,
4. the acknowledgment number is equal to the greatest acknowledgment received on the given connection (TCP.UNA from [RFC793]) and
5. the advertised window in the incoming acknowledgment equals the advertised window in the last incoming acknowledgment.

Alternatively, a TCP that utilizes selective acknowledgments (SACKs) [RFC2018, RFC2883] can leverage the SACK information to determine when an incoming ACK is a "duplicate" (e.g., if the ACK contains previously unknown SACK information).

Triple Duplicate ACK

A triple duplicate ACK is literally the third duplicate ACK (four identical ACKs). See RFC 5681